Conjugated diene polymers have heretofore been broadly used for the modification of resins and in various industrial articles. However, the conjugated diene polymers cause the generation of a gel and discoloration during a long term storage at a high temperature or under a thermal shear stability, so that particularly when it is used for the modification of a resin, the presence of a gel in the polymer and the discoloration of the polymer adversely affects final resin products. Therefore, there has been earnestly desired the appearance of a conjugated diene polymer composition which is excellent in heat stability at a high temperature and shear heat stability and very small in the generation of a gel and discoloration. For these problems, phenol type stabilizers, phosphorus type stabilizers and the like have heretofore been used as stabilizers for preventing gelation.
As a technique concerning a composition of a conjugated diene polymer having an excellent stability and a good color tone, there is a method in which a low molecular weight monophenol type antioxidant is combined with a high molecular weight monophenol type antioxidant and a specific organic carboxylic acid is added (JP-A-3-31,334). There is also a method in which a coupling rubber or a monoalkenyl aromatic compound-conjugated diene block copolymer is combined with a specific phenol type stabilizer or phosphorus type stabilizer (JP-A-4-252,243 and JP-A-4-246,454).
However, these techniques require the use of a large amount of an antioxidant and are not satisfactory in respect of resistance to discoloration. An object of this invention is to solve these problems and provide a rubber-like conjugated diene polymer composition which is suitable as a toughening agent for a styrene resin and excellent in both stability and color tone. In addition, in the prior art, it has been impossible to obtain a styrene resin having an excellent color tone and an excellent impact strength. Accordingly, another object of this invention is to provide a rubber-reinforced styrene resin having an excellent color tone and an excellent impact strength.